For most RF and microwave circuits, only a portion needs to be implemented on a high performance printed circuit board substrate. These high performance substrates are expensive and difficult to process. For economic reasons, board designers prefer to implement as much of the circuit as practical on an inexpensive, easy-to-process substrate and use the high performance substrates only where needed to achieve RF or microwave operation.
As shown in FIG. 1, in one prior art example, the inexpensive substrate, or motherboard 1, has a hole 2 that is slightly larger in area than the high performance substrate, or daughterboard 3. The daughterboard 3 is seated within the hole 2, and the boards 1 and 3 are clamped together with a cast metal shield 4. Signals are routed from one to another with axial lead capacitors 5 which are soldered between the boards 1 and 3. This is not a cost-effective manufacturing process due to the need for the shield 4 and the manual labor required for assembly. The hole 2 also weakens the structural integrity of the motherboard 1. The characteristic impedance of the signal leads 5 is difficult to control. Furthermore, there is poor ground continuity between the motherboard 1 and the daughterboard 3.
FIG. 2 shows another prior art example in which solder paste 6 is manually applied to a motherboard 1 and a daughterboard 3 having substrates with matched thermal coefficients of expansion. In addition to increased manufacturing costs, the connections are prone to bridging because the solder is not shielded from overflow. Furthermore, self-registration of the daughterboard 3 to the motherboard 1 is not possible because there are no contact pads to aid alignment.
Moreover, the solder connections shown in FIG. 2 are provided primarily for ground plane and mechanical interconnection of the motherboard 1 and daughterboard 3. Consequently, these solder connections are few and large, and stress is concentrated at these solder connections. Therefore, if the motherboard 1 and daughterboard 3 were characterized by different thermal coefficients of expansion, the solder connections would be subject to failure when exposed to temperature variations. Accordingly, the prior art example shown in FIG. 2 is not readily susceptible to implementation of an RF or microwave circuit in which printed circuit board substrates having different thermal coefficients of expansion are used.
An efficient printed circuit board design methodology that promotes modular board design while accounting for the daughterboard as a design component is desirable. Also, any difference in the thermal coefficient of expansion for the substrate of the daughterboard with respect to the substrate of the motherboard should not affect the reliability of connections between the boards. The resulting board should also be easy and economical to manufacture.